The present invention relates to a resin composition for bonding the same or different materials selected from glass, ceramic, metals, resins, and the like. More particularly, the present invention relates to a resin composition for high-frequency bonding, which makes possible bonding for a short time by high-frequency heating, without using a heating furnace, and which exhibits high bonding strength even at high temperatures around 50xc2x0 C.
In recent years, since adhesives have remarkably improved performance and function, they have been used in many fields including electric, machinery, civil engineering, architecture, wood, paper, fibers, and chemistry. Adhesives have also been used for the connection of important constituent parts. For example, adhesives have been used even for the portions which are required to have high reliability, such as bonding between break linings of automobiles and honeycomb structural members of airplanes.
Compared with the connection with screws, bolts, nails, welding, or the like, the connection with adhesives has the advantages that the production process can be simplified and the connected portions can be lightened.
When the same or different materials selected from glass, ceramic, metals, resins, and the like, such as used in the fields including automobiles, railroad vehicles, airplanes, and civil engineering works and buildings, are bonded, a primer is often applied to the surface of materials to be bonded prior to the application of an adhesive, in order to improve the adhesion and durability of connection, and after drying and/or curing the primer, an adhesive is applied and cured to bond the materials to be bonded.
In this case, however, a problem arises that it requires much time for the drying of a solvent contained in the primer and/or adhesive or curing reaction. There is also a problem that the vaporization of a solvent has adverse effects on the working environment and the global environment.
To solve the above problems, hot-melt adhesives have recently been utilized in industry. Hot-melt adhesives have the merits that they contain almost or completely no solvent, thereby having small adverse effects on the working environment and the global environment, and having only small fire hazard, and that they are in a solid state at ordinary temperatures, thereby making easy the handling. In addition, hot-melt adhesives can be solidified and bonded by standing to cool after melting and bonding, so that they are suitable for automation, acceleration, and mass treatment.
However, the ordinary hot-melt adhesives have a difficulty that they have low bonding strength at high temperatures and they can often exhibit no desired performance. When hot-melt adhesives with high melting points for the purpose of using at high temperatures are used, a heating furnace is needed when bonding, leading to a high production cost for that. Further, the step of cooling after heating and bonding is needed, rendering a difficulty to the practical application.
Thus, there have been made studies to obtain adhesives having the merits and overcoming the defects of hot-melt adhesives as described above. The resin composition for high-frequency bonding according to the present invention is one of the hot-melt adhesives and has the function of bonding materials to be bonded by heating and melting the composition through the application of a high-frequency voltage with the intervention of the composition between the materials to be bonded, and then by cooling for solidification. The use of an adhesive containing the resin composition for high-frequency bonding according to the present invention requires no heating furnace, so that the equipment for the step of bonding can be made easy and simple as compared with the situation where conventional hot-melt adhesives are used.
The principle of high-frequency heat generation utilized in the present invention is considered by heat generation from molecular friction based on the polarization of molecules. In general, the efficiency of heat generation in thermoplastic resins by the application of a high-frequency voltage is considered excellent for higher dielectric constant (xcex5), higher dielectric loss tangent (tanxcex4), and higher dielectric loss factor {(xcex5)xc3x97(tanxcex4)} as their product. Therefore, thermoplastic resins composed mainly of hydrocarbon bonds seem to be unsuitable for high-frequency heating because of their small molecular polarizability.
For example, for base materials or films made of a polyester, there has been proposed a method in which a resin layer causing dielectric heat generation, such as phenolic resins or urethane resins, is used as a bonding layer and the base materials or films made of a polyester as the materials to be bonded are connected by dielectric bonding, as disclosed in JP-A 56-75824.
For high-frequency dielectric heat generation, JP-A 52-68273 discloses a technique of blending an inorganic material having a high dielectric loss factor in a thermoplastic resin. JP-A 54-161645 discloses a technique of blending a ferroelectric substance in a thermoplastic resin. JP-A 6-228368 discloses a film prepared by blending or applying an electromagnetic wave absorbing substance such as ferrite to a film made of a thermoplastic resin. JP-A 6-228368 discloses a resin composition prepared by blending a liquid polar material adsorbed inorganic porous powder in a thermoplastic resin. JP-B 5-42982, JP-A 2-129243, and JP-A 2-182419 also disclose resin compositions prepared by blending radio-frequency sensitivity improving inorganic compounds, crystal water containing inorganic compounds, and high polar organic materials, respectively, in thermoplastic resins such as polyolefin resins. ANTEC ""91, pp.2245-2247, discloses blending sodium aluminosilicate in ultrahigh molecular weight polyethylene. Microwave sensitizers are commercially available, for example, under FREQUON (trade name) from Struktol Company of America. However, all these techniques have not yet been put into practice because of their low heat generation rate by high-frequency dielectric heating.
For polyester fiber products, methods for obtaining fiber products having a high-frequency fusion property by applying an alkali (earth) metal salt of an acid phosphorus compound or a basic nitrogen-containing compound are disclosed in JP-A 59-15572 or the like. JP-A 59-26576 discloses a method for improving the high-frequency fusion property by applying an emulsion of a high polar polymer such as vinyl chloride or vinylidene chloride to polyester fibers. Polyamide resins have a heat generation property as compared with polyester resins; however, as disclosed in JP-A 62-50122, polyamide resins provide a small temperature difference from materials to be bonded, even by dielectric heating, and therefore, they have insufficient heat generation and bonding properties for putting them into practice as adhesives. As disclosed in JP-A 59 184611, there has been made an attempt to allow the intervention of a metal covered with another adhesive. Further, JP-A 60-130664 discloses a method in which conductive fibers are blended in a polyamide resin and eddy-current loss by high-frequency induction is utilized; however, heat generation rate is low and it requires much time for heating, which is not practical.
An object of the present invention is to provide a thermoplastic resin composition, which is a hot-melt adhesive composed mainly of a thermoplastic resin with heat resistance, and which can achieve heat bonding for a short time by the high-frequency dielectric heating method, and which can exhibit high bonding strength under a high-temperature atmosphere.
The present inventors have extensively studied to attain the above object, and as a result, they have completed the present invention. Thus, the present invention provides a resin composition for high-frequency bonding with a dielectric loss tangent of 0.03 or higher at a frequency of 40 MHz, comprising a thermoplastic resin having a melting point of 80xc2x0 C. to 200xc2x0 C. in an amount of 70% to 99% by volume, based on the total volume of the composition, and a conductive material having a volume resistivity of 10xe2x88x922 xcexa9.cm or lower in an amount of 30% to 1% by volume, based on the total volume of the composition.
In a preferred embodiment, the above thermoplastic resin is a polyolefin resin.
In a preferred embodiment, the above thermoplastic resin is a polyester resin.
In a preferred embodiment, the above thermoplastic resin is a polyamide resin.
In a preferred embodiment, the dielectric loss tangent is 0.05 or higher at a frequency of 40 MHz.
In a preferred embodiment, the above dielectric loss tangent is 0.1 or higher at a frequency of 40 MHz.
In a preferred embodiment, the above conductive material having a volume resistivity of 10xe2x88x922 xcexa9.cm or lower is contained in an amount of 30% to 5% by volume, based on the total volume of the composition.
In a preferred embodiment, the above resin composition for high-frequency bonding exhibits a bonding property by applying a high-frequency voltage to cause heating and melting, and then by cooling for solidification.